We do not have the feature to whitelist the email ID from backend.
In any of your previous campaigns, if the user marked the mail as SPAM or abuse or unsubscribed or the email to that user hard bounced or the user was manually blacklisted then the Email ID is blacklisted from all the Lists. No Broadcast is sent again to that Blacklisted Contact.
Also it is not recommended to whitelist those users and target them again because if the white-listed Email ID gets blacklisted again, this will affect your domain reputation and the mails will start landing in spam in future.